BEIRUT - Islamist rebels waged a counter-offensive in Syria's border town of Albu Kamal on Saturday, challenging the grip of the Islamic State in Iraq and Syria (ISIS) which has seized large areas on each side of the crumbling Iraqi-Syrian frontier.

Earlier this week ISIL fighters appeared to be consolidating their hold over Albu Kamal when the local leader of the rival Nusra Front, al-Qaida's official branch in Syria, pledged allegiance to them.

ISIL is a more radical offshoot of al-Qaida that has its roots in Iraq and expanded into Syria shortly after the start of the three-year insurgency against President Bashar Assad.